Heat index and humidex, but not other measures, identify heat stress compensability during moderate cycling.

Abstract

To examine the suitability of common heat stress indicators, including wet-bulb globe temperature (WBGT), universal thermal climate index (UTCI), heat index (HI), and humidex, to (1) determine uncompensable heat stress and (2) the capability of the indicators to directionally match physiological and perceptual strain in varying thermal environments. Ten healthy, active males (2+/-7 y, 179+/-11 cm, 77.4+/-9.3 kg, 51.9+/-5.3 mL/kg/min) performed two randomized, crossover exercise trials (60-min cycling ~55% ṼO_2peak) in hot-dry (HD, 39.1°C, 33% RH) and warm-humid (WH, 34.5°C, 59% RH) conditions. Metabolic heat production (H_prod), evaporation required for heat balance (E_req), and maximum evaporative rate (E_max) were calculated. Estimated compensability was determined using E_req to E_max ratio. Physiological and perceptual measures occurredthroughout. There were no differences in WBGT (p = .83, d = .59), HI (p = .65, d = 1.9), and humidex (p = .73, d = 1.10) between HD and WH. UTCI in HD was significantly greater than in WH (p $<$ .001, d = 1.19). There were no differences inphysiological measures (p > .34) between HD and WH, indicating different environments with matched WBGT, HI, and humidex matched thermal, cardiovascular, and perceptual strain, but not UTCI. H_prod (p $=$ 0.04, $\eta$ _p ² = .38), E_req (p $<$ 0.001, $\eta$ _p ² = .75), and E_max (p $<$ 0.001, $\eta$ _p ² = .94) in HD were significantly greater than WH. E_req/E_max in both HD and WH indicated uncompensable heat stress. HI and humidex identified compensability, physiological, and perceptual strain adequately under HD and WH conditions.